1. Field of the Invention
The present invention relates to an optical sensor system and method for detecting the presence of hydrides and acid gases, e.g., in semiconductor manufacturing process effluents and semiconductor manufacturing plant environments.
2. Description of the Related Art
In the manufacture of semiconductor products, hydrides and acid gases are frequently used, e.g., as etchants, source reagents, cleaning agents, reactants, chamber cleaning materials, process stream diluents, and treating agents for abatement of effluents of the semiconductor manufacturing plant.
In such applications, there is a need for corresponding monitoring equipment and techniques for detecting the presence of hydrides and acid gases in specific process streams, or gaseous environments of the semiconductor manufacturing facility, to ensure the absence of such gases where they pose a health or safety risk, or are required to be removed in the treatment of gaseous streams containing same.
An illustrative application of such type involves scrubbing of hydrides and acid gases from streams in which they are present, using dry scrubbing apparatus. The dry scrubber apparatus is typically arranged as a single-or multi-vessel installation, in which the scrubber vessel contains a dry scrubber composition such as a chemical sorbent material that irreversibly reacts with the hydrides and acid gases that are present in a gas stream flowed through the vessel.
Since such scrubbing consumes the active dry scrubber medium, it is desirable to monitor the gas discharged from the scrubber vessel. By such monitoring, the approach to “breakthrough” of the hydride and/or acid gas components from the vessel is sensed, and the vessel can be taken off-stream to enable change-out of the scrubber medium, whereby fresh dry scrubber material is provided for renewed on-stream processing of the hydride-and/or acid gas-containing stream.
The point at which the bed of dry scrubber material in the vessel becomes substantially exhausted by cumulative reaction with hydride and/or acid gas components in the gas stream is termed the “end point” of the scrubber material bed, and a corresponding sensor that is arranged to sense the end point is termed an “end point sensor.”
Specific scrubber systems of such type can also include hydride and/or acid gas sensors that are disposed in the bed, e.g., at an intermediate portion thereof, or at scattered multiple sensing locations in the bed of dry scrubber material, to permit the progressive exhaustion of the scrubber material to be monitored, so that the need for change-out of the scrubber medium from the vessel can be more accurately scheduled during the active service life of the dry scrubber material, particularly in instances where the hydride and/or acid gas-containing stream is highly variable in concentrations of such gas stream components.
Conventional scrubber sensors, e.g., end point sensors, include two main categories of devices, electrochemical-based sensors and tape-based sensors.
Electrochemical sensors have two potential shortcomings in use. First, the analyte used to react with the gas stream has a tendency to dry out when used in applications where the gas stream being analyzed is of low relative humidity. Such low relative humidity conditions result in erratic measurements and ultimately sensor failure. Gas streams exhausted from semiconductor manufacturing tools and flowed to scrubbers are typically dry in character, making the use of electrochemical sensors problematic. Second, conventional electrochemical sensors are sometimes cross-sensitive to hydrogen gas (H2), a significant disadvantage since hydrogen is frequently used as a reducing gas in semiconductor processes and correspondingly is present as a common component in gas streams being monitored for hydrides and acid gases.
Tape-based sensors also have inherent limitations when used for scrubber endpoint or general environmental safety monitoring. Such sensor units are characteristically expensive in the first instance, and their cost of ownership (COO) is substantial as a result of having to continually replace color-changing tapes. Such units have movable parts that require maintenance and sometimes result in failure. For example, sensor units of such type are equipped with a pump that draws up to 1 standard liter per minute (slpm) to the sensor tape, for contact therewith so that a color change of the tape indicates the presence of the target gas species. Tape-based sensors also employ electric motors for scrolling the tape after each sampled point. These and associated moving parts have lifetime and reliability issues.
Accordingly, it would be a substantial advance in the art to be able to monitor gas, e.g., a gaseous stream or a gaseous environment, for the presence of hydrides and acid gases, with a monitoring system and methodology that avoids the above-discussed deficiencies of conventional hydride and acid gas sensing technologies.